Process for producing an easily shaped cold-rolled sheet or strip

ABSTRACT

A method for producing a cold-rolled steel sheet or strip with good formability, especially stretch formability, for making pressings with a high buckling resistance from a steel comprising (in % by mass): 0.01 to 0.08% C, 0.10 to 0.80% Mn, maximum 0.15% Si, 0.015 to 0.08% Al, a maximum 0.005% N, 0.01 to 0.04% Ti and/or Nb, whose contents exceeding the quantity necessary for stoichiometric binding of the nitrogen, ranges from 0.003 to 0.015% Ti or 0.0015 to 0.008% Nb, and a maximum 0.15% in total of one or several elements from the group copper, vanadium, nickel, the remainder being iron, including unavoidable impurities, including a maximum 0.08% P and a maximum 0.02% S, comprises preheating the cast slab to a temperature exceeding 1050° C., hot-rolling at a final temperature ranging from over the Ar 3  temperature to 950° C., coiling the hot-rolled strip at a temperature ranging from 550 to 750° C., cold-rolling at a total cold-rolling degree of deformation from 40 to 85%, recrystallization annealing of the cold strip in a continuous furnace at a temperature of at least 720° C., subsequent cooling at 5 to 70 K/s; and skin passing.

This application is a 371 of PCT/EP97/02169 filed Apr. 26, 1997.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing a cold-rolled sheet orstrip of superior strength having good formability especiallystretch-formability for making pressings with a high bucklingresistance.

The pressings are to be of high basic material strength and afteradditional heat treatment as it is usually applied for enamelling, theyare to receive additional bake hardening. In this way, outstandingbuckling resistance characteristics are achieved. For example bodysheets in the motor vehicle industry, such as doors, hoods, roofs, arepressings comprising a high degree of stretch-forming.

In the production of continuous-annealed aluminium killed non-alloyeddeep-drawing steels and which have particular requirements in respect offormability, after cooling from the recrystallization temperature, anadditional annealing, so-called overageing annealing, is applied toensure ageing stability. A non-ageing material is characterized in thateven after extended storage periods no significant changes occur in thematerial's properties and further processing free of stretcher strainand free of defects is possible. In a continuous furnace such treatmentcan take place in an in-line overageing section. In the case of stripwhich is produced in a common hot-coating plant, subsequent externalannealing, usually in the coil, needs to be carried out.Aluminium-killed non-alloyed deep-drawn steels, also called low-carbon(LC) steels, have a carbon content ranging from 0.02 to 0.08%.

Above all in motor vehicle body building, for reasons of weightreduction, the use of the thinnest possible sheet is desired. To providethe required buckling resistance in spite of sheets of reducedthickness, higher strengths are required. Increasingly, bake-hardeningsteels are used for this purpose. Steels with bake-hardening propertiesare characterized by an additional increase in yield strength of thedrawn component. Such an increase is achieved in that the material,apart from the work hardening occurring during pressing, is subjected toan additional increase in strength, the so-called bake hardening. Thephysical reason for this is a carbon-ageing occurring under controlledconditions. Bake-hardening steels and their intended applications alsorequire adequate ageing stability for surfaces free from imperfectionsafter pressing.

In continuous furnaces comprising an in-line overageing section, anon-alloyed LC steel can also be produced as a bake hardening steel, inthat the chemical composition of the steel, the rate of cooling and theoverageing condition are exactly matched to each other. This process isalready used on a commercial scale. Optimization of the productionconditions is for example described by Hayashida et al. (T. Hayashida,M. Oda, T. Yamada, Y. Matsukawa, J. Tanaka: "Development andapplications of continuous-annealed low-carbon Al-killed BH steelsheets", Proc. of the Symp. on High-strength sheet steels for theautomotive industry, Baltimore, Oct. 16-19, 1994, p. 135).

In other processes for producing non-ageing cold-rolled steels with bakehardening properties in continuous strip plants, low-carbon steels,so-called ultra low carbon (ULC) steels are used. A process based on aULC steel for hot-coating plants, partially stabilized with titanium, isdescribed by N. Mizui, A. Okamoto, T. Tanioku: "Recent development inbake-hardenable sheet steel for automotive body panels", Internationalconference "Steel in automotive construction", Wurzburg 24.-26.9.1990).The carbon content is to be between 15 and 25 ppm. The titanium contentis matched to the nitrogen and sulphur contents with 48/14 N<Ti<48(N/14+S/32). The aim is a complete binding of the nitrogen in titaniumnitrides, however a small quantity of carbon must remain soluble toensure the bake-hardening effect takes place. Production in a vacuumdegassing plant is necessary. This process has the advantage thatoverageing annealing can be omitted, thus making it suitable forhot-coating plants. With steels produced in this way, the bake-hardeningparameters determined in tension specimens after 2% initial elongation(BH₂ value) are approx. 40 N/mm². The yield strength is approx. 200N/mm² ; the values for average vertical anisotropy (r value) are approx.1.8.

According to W. Bleck, R. Bode, O. Maid, L. Meyer: "Metallurgical designof high-strength ULC steels", Proc. of the symp. on high-strength sheetsteels for the automotive industry, Baltimore, Oct. 16-19, 1994), forrepresenting such ULC steels partially stabilized with titanium,titanium contents are between 0.6 times and 3.4 times the nitrogencontent. The sum of carbon and nitrogen contents should not exceed 50ppm.

EP 0 620 288 A1 discloses a process for producing steel strip which isonly cold-rolled or hot-coated in continuous strip plants, with thissteel strip apart from ageing stability also comprising highbake-hardening characteristics and good deep-draw characteristics due tohigh r values. A ULC steel on its own or a ULC steel either with atitanium alloy or an niobium alloy is annealed above the Ac₃transformation temperature, i.e. in the austenitic range. In thisprocess, the bake-hardening values attain 100 N/mm². No overageingannealing is necessary. As this is a ULC steel, steel production musttake place in a vacuum degassing plant. The high annealing temperaturesnecessary with this process create difficulties regarding stripflatness. Application of this process on a commercial scale is notknown.

Bleck et al. (op. cit.) point out that the production of a non-ageingsteel with good shaping characteristics based on non-alloyed LC steels,is not possible without overageing, in continuous strip plants. Sincethe cooling process in hot-coating plants in current use is limited dueto the hot-dip galvanizing setup, in-line overageing annealing asmentioned above cannot take place. Consequently, with the known state ofthe art, the production of non-ageing steels with bake-hardeningproperties, in hot-coating plants, is exclusively limited to ULC steels.Thus the processes, applied so far or described in the literature, forproducing in continuous strip plants, cold-rolled sheet with goodformability and which comprises bake-hardening properties, eithernecessitate the additional annealing treatment as described above (if asoft non-alloyed Al-killed deep-drawn steel is used), with such aproduction not being possible in a hot-coating plant; or else theynecessitate the use of ULC steels of very low carbon content, with thesesteels being more expensive to produce. The processes described abovebased on ULC steels mainly comprise steels with yield strength in thelower region up to 240 N/mm². Due to the high average r values (>1.5)they are used for pressings with a high degree of deep drawing.

SUMMARY OF THE INVENTION

It is thus the object of the present invention to produce, in acontinuous strip plant without subsequent overageing-annealingtreatment, a non-ageing cold-rolled steel sheet or strip of superiorstrength with good formability and with a high buckling resistance; withthe said sheet or strip also comprising good bake-hardening properties.The combination of high basic material strength and bake-hardeningpotential is to provide the pressings with excellent resistance tobuckling.

This object is met by a method for producing a cold-rolled sheet orstrip with good formability, and especially stretch-formability, formaking pressings with a high buckling resistance from a steel comprising(in % by mass):

0.01-0.08% C

0.10-0.80% Mn

max. 0.60% Si

0.015-0.08% Al

max. 0.005% N

0.01-0.04% Ti and/or Nb whose contents exceeding the quantity necessaryfor stoichiometric bonding with nitrogen, ranges from 0.003 to 0.015% Tior 0.0015 to 0.008% Nb,

max. 0.15% in total of one or several elements from the group copper,vanadium, nickel, the remainder being iron, including unavoidableimpurities,

including max. 0.08% P and max. 0.02% S, with the following steps:

preheating the cast slab to a temperature exceeding 1050° C.;hot-rolling at a final temperature ranging from over the Ar₃ temperatureto 950° C., preferably ranging from 870 to 950° C.; coiling thehot-rolled strip to a temperature ranging from 550 to 750° C.;cold-rolling with a total degree of deformation from 40 to 85%;recrystallization annealing of the cold strip in a continuous furnace ata temperature of at least 720° C. with subsequent cooling with highcooling rates of 5 to 70 K/s; and then skin passing.

The steel's non-ageing properties are achieved by an addition oftitanium which is matched to the nitrogen content. This results in anearly complete binding of the nitrogen, an element known tosignificantly influence ageing stability. In the ageing tests (seeexamples below) it was found that ageing stability is adequate when aquantity of titanium is present which exceeds the quantity of titaniumin nitrogen binding, thus ensuring the formation of a minimum quantityof titanium carbides. So as to provide the steel with the strengtheningcharacteristics necessary for the high degree of deformation, andadequate elongation and ductility characteristics, the volume and numberof titanium carbides must however not be too high. Thus the quantity ofthe nitride-forming agent not bound to nitrogen should be 0.003 to0.015% Ti or 0.0015 to 0.008% Nb. This limitation of the percentage ofnitride forming agents ensures even mechanical properties which arelargely invariable to process-bound fluctuations in hot-striptemperature control (influencing the precipitation distribution).

The application of this analysis concept ensures the presence ofsufficient dissolved carbon, after cooling from the recrystallizationtemperature, for good bake-hardening properties.

Together with, or instead of, titanium as a micro alloy element, niobiumcan also be used for nitride and carbide formation.

For hot galvanized sheet, the silicon content should preferably belimited to max. 0.15%.

The method according to the invention has the economic advantage ofomitting the additional process step of overageing annealing to achieveageing stability, although the steel composition is based on theanalysis of soft non-alloyed Al-killed (LC) steels. Due to this analysisconcept, steel production can take place without expensive metallurgicalproduction processes. In addition, only small quantities of titanium orniobium are required; as a result the steel can also be economicallyproduced from the point of view of alloying additions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the differential strengtheningindex versus the total elongation for steel having a coiling temperatureof 730° C.

FIG. 2, is a graphical representation of the differential strengtheningindex versus the total elongation for steel having a coiling temperatureof 600° C.

DETAILED DESCRIPTION OF THE INVENTION

The method comprises the following steps:

preheating the cast slab to a temperature exceeding 1050° C.;

hot-rolling at a final temperature ranging from >Ar₃ to 950° C.;

coiling the hot-rolled strip in a temperature range of 550 to 750° C.;

cold-rolling with a total cold-rolling degree of deformation from 40 to85%;

recrystallization annealing of the cold strip in a continuous furnace ata temperature of at least 720° C.;

subsequent cooling at 5 to 70 K/s; and

skin passing.

Preferably, the cold strip is heated to the temperature ofrecrystallization annealing at a rate of 5 to 10 K/s. Preferably,recrystallization annealing takes place in-line in a zinchot-galvanizing plant.

The steel strip or sheets produced according to the invention arecharacterized by a high initial yield strength (exceeding 240 N/mm²) anda high strengthening ability in the range of small plastic elongation.Together with low values of vertical anisotropy which indicate apreferred flowing from thickness, a high degree of stretch-forming inpressings makes these ideal for automotive application, e.g. automotivebody parts. The significant strengthening of this material which alreadyoccurs with small plastic deformation and which manifests itself in veryhigh work-hardening values, constitutes a significant factor in thecharacteristics of this product. The significant strengtheningencourages load transmission to adjacent areas of the material, thusavoiding early local material failure, e.g. constriction. Thus thematerial can flow more evenly across the entire surface of the pressing.In addition, the small variations in the r values depending on the angleto the rolling direction encourage an even deformation behavior. Thisisotropic behavior is upheld by small values in the planar anisotropy.

EXAMPLE

The slabs made by continuous casting of the steels A and B producedaccording to the invention, whose chemical compositions are shown inTable 1, were reheated in a pusher-type heating furnace to temperaturesof approx. 1200° C. and hot-rolled above the Ar₃ temperature to finalthicknesses of 2.8-3.3 mm. The final rolling and coiling temperaturescan be seen from Table 2. For the strip of the steels A and B, twocoiling temperature classes were used: 730° C. (Steels A1 and B1) and600° C. (Steels A2 and B2). The strips were cold-rolled to thicknessesbetween 0.8 and 1.0 mm with degrees of deformation between 65 and 75%and subsequently in a hot-coating plant they were first subjected torecrystallization annealing and then zinc coated by hot-dipgalvanization. The strip temperature in the recrystallization furnacewas 800° C. The cooling rates after recrystallizing annealing werebetween 10 and 50 K/s. The zinc coated strips were skin pass rolled at1.8% and after that were free of yield strength elongation.

Tables 2 and 3 show the mechanical characteristics and grain sizes,determined during tension tests, of the strips A and B, measured at anangle of 90° to the direction of rolling. Only the r values and thevalues for the planar anisotropy are calculated as follows, in eachinstance from three tension specimens which were derived in the angularpositions of 0°, 45° and 90° to the direction of rolling

r_(m) =(r₀ °+2 r₄₅ °+r₉₀ °)/4,

Δr=(r₀ °-2 r₄₅ °+r₉₀ °)/2.

The BH₀ value corresponds to the increase in the lower yield strengthafter heat treatment of 20 minutes at 170° C. The value WH indicates theextent of work hardening at a stretching of the tension specimen by 2%The amount is calculated by subtracting the yield strength Rp₀.2 fromthe tension measured at 2% deformation. The value BH₂ corresponds to therise of the lower yield strength after heat treatment of 20 minutes at170° C., measured at the tension specimen pre-stretched by 2%.

After artificial ageing of 60 minutes at 100° C., the zinc hot dipgalvanized cold-rolled strips from steels A and B show a nearlyunchanged level of the lower or upper yield strength (Table 3). Theshaping of the yield strength too remains below 0.5% as a result ofwhich ageing stability for processing free of stretch strains isadequate even after extended storage periods. The curve of thedifferential (momentary) strengthening index (n value) above totalelongation is shown in FIG. 1 for steel A1 (coiling temperature 730° C.)and in FIG. 2 for steel A2 (coiling temperature 600° C.). The maximumsof the differential n values are shown in Table 2; with the steels A andB they attain at least 0.170 for both coiling temperature classes; inthe case of high coiling temperatures even a minimum of 0.180. The nvalue maximum of the steels A and B is in the range of little overallexpansion, between 2 and 5%. For the higher-coiled variants A1 and B1,the yield strength are approx. 50 N/mm² higher than for the low-coiledvariants A2 and B2, so that the initial position of the yield strengthcan be determined by selecting the coiling temperature. The values forthe average vertical anisotropy of the steels A1, A2, B1 and B2according to the invention are a low 1.0-1.1. Irrespective of thecoiling temperature, they have isotropic characteristics with Δr valuesbetween 0 and 0.3. When using the high coiling temperatures, the workhardening values which represent a measure of the strengthening byplastic deformation, are very high at approx. 50 N/mm². Irrespective ofthe coiling temperature, the parameters for bake-hardening with orwithout initial forming reach at least 45 N/mm² in all cases. Theincrease in the yield strength after painting a pressed component can beestimated by the sum WH+BH₂. In the case of the high coilingtemperatures (steels A1 and B12), these values are at least 100 N/mm².In the case of the lower coiling temperatures (steels A2 and B2) the sumWH+BH₂ is still favorable, being at least 60 N/mm².

Tables 1, 2 and 3 additionally show steels C to E for comparison. Bycontrast to the steels A and B, these steels either contain no titanium(steel E) or else comprise titanium contents which aresub-stoichiometric in respect of the nitrogen content (steels C and Dwith Ti/N<3.4). The values of the initial condition, i.e. non-aged,refer to the skin pass rolled condition. In the case of these comparisonsteels, the rise of the lower yield strength (R_(el)) and the yieldstrength elongation after artificial ageing are significantly higherthan with the steels A and B produced according to the invention. Aboveall the upper yield strength (R_(eh)) increases up to 70 N/mm².Fault-free processing after extended storage is not possible in the caseof steels C to E.

Steel F does not contain any titanium but niobium. Due to the coilingtemperature of 600° C. and the alloying with niobium, its yield strengthis very high at 350 N/mm². The average r value is 1.0 and the Δr valueat -0.20 is favorable for even formability behavior. As is the case withsteels A and B which are titanium alloyed, with the Nb-alloyed steel F,the lower and upper yield strength are also stable and the yieldstrength elongation is below 1% so that here too, processing free of anystretch strains is possible after extended storage periods of thematerial.

The formability of steels A1 and B1 produced according to the inventionwas comprehensively examined in a large-scale trial under near-practicalconditions, using press-moulded passenger motor vehicle bonnets. Inregard to the pressings maintaining their shape and surface, excellentresults were achieved which were reproducible during processing evenafter a storage period of 5 months.

                                      TABLE 1                                     __________________________________________________________________________    Steel                                                                             C   Mn Si  P  S   Al N   Ti Nb  Ti/N                                      __________________________________________________________________________    A   0.042                                                                             0.24                                                                             0.01                                                                              0.009                                                                            0.005                                                                             0.037                                                                            0.0028                                                                            0.016                                                                            --  5.7                                       B   0.041                                                                             0.24                                                                             0.05                                                                              0.009                                                                            0.005                                                                             0.042                                                                            0.0025                                                                            0.015                                                                            --  5.0                                       C   0.050                                                                             0.25                                                                             0.01                                                                              0.009                                                                            0.010                                                                             0.030                                                                            0.0042                                                                            0.009                                                                            --  2.1                                       D   0.044                                                                             0.26                                                                             0.01                                                                              0.011                                                                            0.007                                                                             0.038                                                                            0.0034                                                                            0.009                                                                            --  2.6                                       E   0.031                                                                             0.23                                                                             0.01                                                                              0.010                                                                            0.011                                                                             0.039                                                                            0.0045                                                                            -- --  --                                        F   0.062                                                                             0.71                                                                             0.01                                                                              0.016                                                                            0.005                                                                             0.043                                                                            0.0064                                                                            -- 0.022                                                                             --                                        __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                   Thick-                                                            Final   Degree                                                                            ness                                                              rolling                                                                           Coiling                                                                           of cold                                                                           of cold                 Grain                                     temp.                                                                             temp.                                                                             rolling                                                                           strip                                                                             Rp.sub.0.2                                                                         Rm   A  Average                                                                              size in                                Steel                                                                            (° C.)                                                                     (° C.)                                                                     (%) (mm)                                                                              (N/mm.sup.2)                                                                       (N/mm.sup.2)                                                                       (%)                                                                              r value                                                                           Δ r                                                                        μm.sup.2                            __________________________________________________________________________    A1 910 730 70  1.0 262  375  33 1.1 0.25                                                                             180                                    A2 870 600 70  1.0 315  390  35 1.0 0.18                                                                             130                                    B1 900 730 73  0.8 265  375  31 1.0 0.28                                                                             170                                    B2 870 600 70  1.0 318  395  34 1.1 0.15                                                                             130                                    C  870 570 61  1.5 285  373  33                                               D  880 600 65  1.0 298  390  33                                               E  900 760 68  0.9 232  365  32        250                                    F  890 600 65  1.0 350  423  33 1.0 -- 100                                                                        0.20                                      __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Ageing characteristics, work and bake-hardening values of the steels          examined                                                                         ΔR.sub.el after                                                              ΔR.sub.eh after                                                              ΔRe after                                                     ageing                                                                             ageing                                                                             ageing                                                                             WH   BH.sub.0                                                                           BH.sub.2                                          Steel                                                                            (N/mm.sup.2)                                                                       (N/mm.sup.2)                                                                       (%)  (N/mm.sup.2)                                                                       (N/mm.sup.2)                                                                       (N/mm.sup.2)                                                                       η.sub.max                                                                    ε.sub.nmax (%)                                                             Remark                               __________________________________________________________________________    A1 0    3    <0.5 51   63   65   0.187                                                                            3.0  Invention                            A2 0    2    <0.5 11   45   53   0.171                                                                            3.5  Invention                            B1 1    3    <0.3 44   61   58           Invention                            B2 2    3    <0.5 20   41   52           Invention                            C  14   63   3                           Comparison                           D  17   55   3                           Comparison                           E  21   46   2.5                         Comparison                           F  0    1    <0.5 33   46   47           Invention                            __________________________________________________________________________     Tensile tests were carried out on specimens measuring 80 mm in length.        "ΔR.sub.el after ageing" indicates the increase in the lower yield      strength after artificial ageing of the tension specimens (100° C.     60 minutes).                                                                  "ΔR.sub.eh after ageing" indicates the increase in the upper yield      strength after artificial ageing of the tension specimens (100° C.     60 minutes).                                                                  "ΔRe after ageing" indicates the yield strength elongation after        artificial ageing of the tension specimens (100° C., 60 minutes).      "WH" indicates workhardening after 2% stretching.                             "η.sub.max " indicates the maximum differential n value.                  "ε.sub.nmax " indicates the degree of total elongation where the      maximum n value occurs.                                                  

What is claimed is:
 1. A method for producing a cold-rolled steel sheetor strip having good formability, including stretch-formability, formaking pressings with a high buckling resistance from a steel comprising(in % by mass):
 0. 01 to 0.08% C0.10 to 0.80% Mn maximum 0.15% Si 0.015to 0.08% Al maximum 0.005% N 0.01 to 0.04% Ti and/or Nb, whose contentsexceeding the quantity necessary for stoichiometric binding of thenitrogen, ranges from 0.003 to 0.015% Ti or 0.0015 to 0.008% Nb, and amaximum 0.15% in total of one or several elements from the group copper,vanadium, nickel, the remainder being iron, including unavoidableimpurities, including a maximum 0.08 % P and a maximum 0.02% S; themethod comprising:preheating the cast slab to a temperature exceeding1050° C.; hot-rolling at a final temperature ranging from over the Ar₃temperature to 950° C.; coiling the hot-rolled strip at a temperatureranging from 550 to 750° C.; cold-rolling at a total cold-rolling degreeof deformation from 40 to 85%; recrystallization annealing of the coldstrip in a continuous furnace at a temperature of at least 720° C.;subsequent cooling at 5 to 70 K/s; and skin passing.
 2. The methodaccording to claim 1 wherein the cold strip is heated to the temperatureof recrystallization annealing at a rate ranging from 5 to 10 K/s. 3.The method according to claim 1 wherein recrystallization annealing ofthe cold-rolled strip takes place in-line in a zinc hot-dip galvanizingplant.
 4. The method according to claim 1 wherein hot rolling takesplace at a final temperature ranging from 870 to 950° C.